Evolutionary trends. Circulation in jawed vertebrates
Although clearly related to its mode of life, the blood
system of a species also reflects its evolutionary history.
The most significant change that occurred during early
vertebrate evolution was the appearance of animals that
could live and breathe on land. The first of these were
the amphibians. Reptiles became even more independent
of water because of their waterproof skins and shelled
eggs, and from them evolved the most sophisticated land
vertebrates, the mammals and birds. Obtaining oxygen
entirely from air, instead of from water, involved drastic
changes in the circulatory system.
Land vertebrates use their lungs to exchange carbon
dioxide for oxygen from the air. Lungs may have evolved
from a structure in fishes called the swim bladder,
a sac that grows out from the anterior part of the gut.
Fishes use it for buoyancy control, but it is possible
that it was originally useful as an accessory for respiration.
The problem is that lungs are found at a different site
in the circulatory system from that of the gills, where
oxygenation occurs in fishes. Instead of circulating
around the body, as it does in fishes, oxygenated blood
from the lungs returns to the heart. Therefore, in evolutionary
terms, if mixing of oxygenated and deoxygenated blood
was to be avoided in the heart, alterations to its structure
had to occur. Land vertebrates developed lungs, a new
vein (the pulmonary vein) to take blood from them to
the heart, and a double circulation, whereby the heart
is effectively divided into two halves—one-half concerned
with pumping incoming deoxygenated blood from the body
to the lungs and the other with pumping oxygenated blood
from the lungs around the body.
There are also modifications in the arterial and venous
systems related to the appearance of lungs in the circulation.
In the venous system, the paired posterior cardinal
veins are replaced by a single posterior vena cava,
and the renal portal system disappears. The main modifications
to the basic arterial pattern involve what are the gill
arteries of fishes. The anterior of these became responsible
for carrying oxygenated blood to the head and to the
brain; the intermediate arteries for carrying blood
to the dorsal aorta, and so around the body; and the
posterior arteries for carrying deoxygenated blood to
the lungs.
In fishes the four chambers of the heart are all well
developed. Blood passes in sequence through the sinus
venosus, atrium, ventricle, and conus arteriosus. The
ventricle is the main pumping chamber, as it is in the
hearts of all land vertebrates. During the evolution
of the heart, the ventricle and atrium came to predominate;
the sinus venosus became part of the atrium, while the
conus arteriosus was incorporated into the ventricle.
The atrium itself became a double structure—the two
auricles—as did the ventricle, but the conversion of
the ventricle into two chambers occurred later in evolution
than the division of the atrium.
